Time-resolved optical sensors have become important for various applications, such as PET, FLEM and LiDAR. These image sensor devices, when implemented in CMOS, generally comprise avalanche photodiodes operating in Geiger mode, known as single photon avalanche diodes (SPAD), and time-to-digital converters which determine the time of arrival of photons. The photodiodes are arranged in an array of a sensor pixel area. One or more photodiodes are each coupled with one time-to-digital converter, so that in time-correlated operation mode, the timing of incoming photons which are periodically generated by a laser can be precisely measured so that an accurate 3D image reconstruction of the scanned scene can be obtained.
Time-of-flight image sensors require high levels of accuracy and precision on time quantization typically using time-to-digital converters. Therefore, the availability of precise and uniform timing information over the sensor pixel area is required due to non-linearity and jitter.
In a most common case, a single time-to-digital converter is used together with a distribution network as e.g. shown in Carimatto, A., et al., “A 67,392-SPAD PVTB-Compensated Multi-Channel Digital SiPM with 432 Column-Parallel 48 ps 17 b TDCs for Endoscopic Time-of-Flight PET”, ISSCC 2015. As such a single time-to-digital converter requires to be used together with a distribution network, high power consumption and non-linearity are imposed as multiple phases of an oscillator or the time-to-digital converter need to be routed over the sensor area.
A further approach provides one time-to-digital converter per pixel which can lead to a lower power consumption, in case of a photon-starved application. However, due to its event-driven operation those time-to-digital converters are required to operate in open loop and independently from each other, which is highly susceptible to process-voltage-temperature variations, jitter accumulations and non-linearity, as e.g. shown in Richardson, J., et al., “A 32×32 50 ps resolution 10-bit time to digital converter array in 130 nm CMOS for time correlated imaging”, CICC 2009.
Due to area constraints, typical oscillators for time-to-digital converters are formed by ring oscillators, such as known from P. Kinget, “Integrated GHz voltage-controlled oscillators”, Springer, 1999.
Multiple oscillator coupling is used in many fields and applications as generally known from Galton, I., et al., “Clock distribution using coupled oscillators”, ISCAS 1996. Due to uncorrelated phase noise in different oscillators, the phase noise of a system with coupled oscillators can be significantly reduced.
It is an object of the present invention to provide an oscillator arrangement for time-to-digital converters of an image sensor device which provides high accuracy timing information over the sensor pixel area.